USRP + RFX2400 - Emitting power control


#1

Hello everybody,

I am currently working with USRP and RFX2400, and I would like to have
accurate control over the power that I am emitting.
Actually, I have seen that:
-> the gain parameter does not have any impact
(gain_range[gmin,gmax,step] = [0,0,1])
-> the power level is somehow controlled by the amplitude parameter

And my questions are:

  • What is the ““exact maximum”” of the amplitude to reach the specified
    maximal power of 17dBm (50mW)? (Is that 16000?)
  • By the way, what is the relationship between amplitude and power? Is
    “power = coefficient * (amplitude)2”? Or is the power only proportional
    to what the board can/decides to provide as Voltage * Intensity at the
    antenna?
  • In the o’scope utility provided by the sources, what are the two
    signals I can observe when using 1 card/1 antenna? Are these two signals
    Q and I? What are the y-axis units? Is it an expression of the power or
    of the amplitude (like sqrt(2)*amplitude, but with one zero missing)?

Thank you in advance for your support,
Arthur Schmitt


#2

Arthur wrote:

And my questions are:

  • What is the ““exact maximum”” of the amplitude to reach the specified
    maximal power of 17dBm (50mW)? (Is that 16000?)

You need to understand the realities of practical RF systems. The gain
and maximum power will vary:

  • with different modulation systems
  • from unit to unit
  • over temperature
  • over frequency
  • with how well matched your antenna is
  • with how much compression you are willing to tolerate

This is not just true for the RFX2400, but all RF systems.

It is impossible to say that maximum power exactly some level,or that
maximum power occurs at a specific baseband amplitude. All of those
factors come into play. If you absolutely must get the maximum power,
then you need to calibrate the power vs. amplitude in your particular
situation. The other option is to choose an amplitude that produces
good signal levels across a wide range of situations. 10,000 to 16000
is usually ok.

  • By the way, what is the relationship between amplitude and power? Is
    “power = coefficient * (amplitude)2”? Or is the power only proportional
    to what the board can/decides to provide as Voltage * Intensity at the
    antenna?

Yes, power will be proportional to the square of the baseband
amplitudes. If you double the amplitude, you will get 6dB more output
power, unless you are in compression.

  • In the o’scope utility provided by the sources, what are the two
    signals I can observe when using 1 card/1 antenna? Are these two signals
    Q and I? What are the y-axis units? Is it an expression of the power or
    of the amplitude (like sqrt(2)*amplitude, but with one zero missing)?

Those are amplitudes of the complex quadrature baseband received
signals.

Matt


#3

And my questions are:

  • What is the ““exact maximum”” of the amplitude to reach the specified
    maximal power of 17dBm (50mW)? (Is that 16000?)

Hi,
I have read somewhere in the archive of the mailing list a value of
32767
for the max amplitude! Right?!
Chris